1. Field of the Invention
This invention relates to energy storage and conversion devices, and in particular, to energy conversion and storage devices using thin film oxide and non-oxide electrodes manufactured by thermal spray.
2. Description of the Related Art
Energy storage devices, such as batteries and supercapacitors, and energy conversion devices, such as fuel cells and thermoelectrics, both require electrodes comprising an active material for the energy storage, conversion, and/or release processes. Each year, billions of dollars are spent on both primary and rechargeable batteries for use in applications ranging from small batteries for portable electronics and communications equipment, to larger batteries used for automobiles and uninterruptible power supplies (UPS).
The LiSi/FeS2 couple is the primary power source used for thermally activated batteries (“thermal batteries”) for some nuclear weapons and missiles, as described in U.S. Pat. Nos. 4,119,769, 4,840,859, and 4,675,257, which are incorporated by reference herein. These batteries are designed to function only when the electrolyte phase becomes molten. Until the internal pyrotechnic heat source is ignited, the batteries are inert and have an almost unlimited shelf life. Common electrolytes used for these applications include the LiCl—KCl eutectic that melts at 352° C. and the all-lithium LiCl—LiBr—LiF minimum-melting electrolyte that melts at 436° C. The cathode, separator, and anodes for thermally activated batteries are prepared by cold pressing of powders in dies. The separator contains enough MgO (typically, 35 weight %) to act as an immobilization agent for the electrolyte once the battery has been activated and the electrolyte melts. The catholyte contains 25% or more of separator material and, in many cases, 1.5% Li2O to act as a lithiation agent to mitigate voltage transients caused by electroactive iron impurities. The anode contains 20-25% electrolyte to aid in pelletizing and to improve the electrochemical performance by increasing the ionic conductivity. Each cell in a bipolar thermal-battery stack contains pellets of anode, separator, cathode, and pyrotechnic source (typically, Fe/KClO4 blends) and 304 stainless steel current collectors between the anode and the heat pellet and between the heat pellet and cathode of the adjacent cell. These also serve as thermal buffers to moderate the heat input to the active cell components. This is important for the FeS2 (pyrite) cathode, in that FeS2 becomes thermally unstable above 550° C., decomposing according to equation 1:2FeS2⇄2FeS+S2(g)  [1]Under these conditions, the fugitive sulfur vapor can react with the LiSi anode to generate enough heat to cause a thermal-runaway condition, where the battery self-destructs.
The need to press catholyte powders such as FeS2 into thin films or pellets for use in thermal batteries increases production costs because of the high labor costs associated with processing of the material (e.g., blending, pelletizing, and quality control checks for weight and thickness). While the current technology of using cold-pressed pellets is suitable for its intended purposes, it has a number of intrinsic limitations. The thinnest pellets that can be fabricated with reasonable yields are in the range from 0.010 to 0.012 inches in thickness. For many applications this results is far greater capacity than is actually needed. The use of a graphite-paper substrate as a reinforcing agent greatly helps with the cathode pellet, but is not an option with the separator and anode pellets, however. The use of excess material increases the length and mass of the thermal battery unnecessarily. Pressing of large pellets becomes increasingly more difficult as the diameter of the pellet is increased from 1 inch to 5 inches. The necessary pressure for compaction of pellets increases rapidly as the square of the area of the pellet, so that presses with capacities of 500 tons or more are needed for the larger pellets. Such large presses are very expensive.
Thin films, and thin film electrodes in particular have been fabricated by other techniques, including spray pyrolysis and chemical vapor deposition (CVD). Spray pyrolysis has been used to fabricate electrodes comprising LiCoO2, LiMn2O4, and yttria stabilized zirconia (YSZ). CVD has been used to fabricate electrodes comprising MoS2 (by conventional CVD), ZrO2—TiO2—Y2O3 (by laser CVD, wherein the laser is the heat source of the substrate and reaction activator), and TiS2 (by plasma CVD). Thin film electrodes have also been prepared by sol-gel methods (CeO2—TiO2 electrodes), electrochemical method (amorphous MnO2 electrodes), and molecular beam deposition (γ—In2Se3). An approach to fabrication of electrodes by thermal spray has been reported by R. Henne (Institute fÅr Technische Thermodynamik, Stuttgart, Germany) for at least one energy conversion device, a solid oxide fuel cell, wherein yttria-stabilized zirconia (YSZ) and porosity-graded perovskite deposited by DC are thermal sprayed to fabricate multilayer structures. R. Zatorski of Sulzer-Metco has also reported production of battery electrodes by thermal spray techniques. However, the above reports are directed to thermally stable materials which do not decompose at the high temperatures generally required for thermal spray.
Synthesis of thin films of pyrite in particular has previously been investigated. G. Pimenta et al. have produced pyrite using H2S-reactive iron. Pyrite and pyrite films have also been prepared by chemical vapor transportation, sulfurization of iron oxides, electrodeposition of iron films, argon and reactive sputtering, screen printing processes, and chemical vapor deposition. Conventional and fine pyrite (micron-sized) is also produced in aqueous solution. Nonetheless, despite the variety of methodologies available for the production of thin films, there remains a need for energy storage and energy conversion devices with effective pyrite couples that are efficiently and easily produced.